To attain a deeper understanding of behavioral development, we use Norway rats, domesticated mice, and transgenic mice in a variety of psychobiological investigations that address several levels of analysis. Reductionistic analyses involve thermotaxis and geotaxis, two fundamental elements in early postnatal behavior. These elements are studied developmentally -- separately and then in combination so we can analyze how they are integrated during early development; we apply a variety of experimental methods, including pharmacological and genetic manipulation of brown adipose tissue thermogenesis in combination with a novel modeling approach. Suckling, an early adaptive system, is examined as a learned, congenital behavior, acquired through stimulation intrinsic to birth process. We simulate separate components of the birth process and test hypotheses about how the stimuli function to condition fetus. The postnatal expression of this learning is then studied. On a more molar level, we explore interrelations among suckling behavior, satiety, and induced perceptual references. We examine the role of oxytocin as a common pathway or peptide mechanism in homeostasis and reward. Modeling approaches are again introduced in the analysis of regulation by individuals and groups. We also examine complex behavioral processes during development, as a means of understanding the rules of interaction and the basis of developmental synchrony. Weaning, a universal transition in mammalian ontogeny, provides a model of the development of independence from the nest, as well as emergent sociality. Mother-litter and intra-litter interactions are studied both experimentally and with computational methods involving our individual-based modeling approach. These studies integrate with our new knowledge concerning oxytocinergic mechanisms. We continue our studies of maternal behavior by elucidating the sudden, prepartum rise in maternal responsivity to infant ultrasonic vocalization, using a series of playback, endocrine, and experiential studies. We also initiate mouse research, for this will provide a meaningful and valuable connection from psychobiology to the field of molecular genetics. The mouse experiments are both parallel to and integrated with the rat data. Genetically-engineered mutants are used to test specific hypotheses and concepts. These experiments represent both a continuation of past research and a major, integrative effort involving new methods and approaches.